Carrier aggregation is one of the new features recently developed by the members of the 3rd-Generation Partnership Project (3GPP) for so-called Long Term Evolution (LTE) systems, and is standardized as part of LTE Release 10, which is also known as LTE-Advanced. An earlier version of the LTE standards, LTE Release 8, supports bandwidths up to 20 MHz. In LTE-Advanced, bandwidths up to 100 MHz are supported. The very high data rates contemplated for LTE-Advanced will require an expansion of the transmission bandwidth. In order to maintain backward compatibility with LTE Release 8 user terminals, the available spectrum is divided into Release 8-compatible chunks called component carriers. Carrier aggregation enables bandwidth expansion beyond the limits of LTE Release 8 systems by allowing user terminals to transmit data over multiple component carriers, which together can cover up to 100 MHz of spectrum. Importantly, the carrier aggregation approach ensures compatibility with earlier Release 8 terminals, while also ensuring efficient use of a wide carrier by making it possible for legacy terminals to be scheduled in all parts of the wideband LTE-Advanced carrier.
The number of aggregated component carriers, as well as the bandwidth of the individual component carrier, may be different for uplink (UL) and downlink (DL) transmissions. A symmetric configuration refers to the case where the number of component carriers in downlink and uplink is the same. An asymmetric configuration refers to the case where the number of component carriers is different. The number of component carriers configured for a geographic cell area may be different from the number of component carriers seen by a given terminal. A user terminal, for example, may support more downlink component carriers than uplink component carriers, even though the same number of uplink and downlink component carriers may be offered by the network in a particular area.
LTE systems can operate in either Frequency Division Duplex (FDD) mode or Time Division Duplex (TDD) mode. In FDD mode, downlink and uplink transmissions take place in different, sufficiently separated, frequency bands. In TDD mode, on the other hand, downlink and uplink transmission take place in different, non-overlapping time slots. Thus, TDD can operate in unpaired spectrum, whereas FDD requires paired spectrum.
TDD mode allows for different asymmetries in terms of the amount of resources allocated for uplink and downlink transmission, respectively, by means of different downlink/uplink configurations. These differing configurations allow the shared frequency resources to be allocated to downlink and uplink use in differing proportions. Accordingly, uplink and downlink resources can be allocated asymmetrically for a given TDD carrier.
To avoid severe interference between downlink and uplink transmissions among different cells, neighbor cells should generally have the same downlink/uplink configuration. If this is not done, uplink transmission in one cell may interfere with downlink transmission in the neighboring cell (and vice-versa). Hence, downlink/uplink asymmetry can typically not vary between carriers/cells, but is signaled as part of the system information and remains fixed for a long period of time.
One consideration for carrier aggregation is how to transmit control signaling on the downlink from the wireless base station (an “eNodeB” or “eNB” in 3GPP terminology) to a mobile terminal (a “user equipment” or “UE” in 3GPP terminology). Downlink control signaling may include downlink assignment and uplink grant information (collectively referred to as “resource grant information” herein) as well as acknowledgement (ACK) signaling for hybrid automatic repeat request (Hybrid-ARQ, or HARQ) protocols. One solution is to transmit all types of downlink control information on multiple downlink component carriers associated with different uplink component carriers. However, additional control signaling issues arise when uplink and downlink resources are asymmetrical, whether in terms of uplink/downlink carriers or uplink/downlink subframes, or both.